This invention generally relates to the use of TiB.sub.2 and other refractory hard metal alloys (hereinafter referred to collectively as RHM), as an electrode surface in an aluminum reduction cell.
Ransley et al (U.S. Pat. No. 2,756,743) were probably the first to appreciate the utilization of RHM as cathode materials in an aluminum reduction cell. RHM in pure form are very resistant to the molten aluminum and cryolite found in an aluminum reduction cell and moreover generally have higher conductivities than the conventional carbon products used in a reduction cell. In addition, RHM and in particular TiB.sub.2 are readily wet by molten aluminum, whereas the carbon products normally used are not.
Although the use of RHM in aluminum reduction cells was conceptually a significant improvement, such use was fraught with practical problems and as a result the development of RHM cathodes has never met with any significant commercial success.
One major problem faced by the workers in this area was the deleterious effects of oxide in the RHM shapes used in the reduction cell. Normally the RHM shapes were formed from RHM powder by either hot pressing or cold pressing and sintering. However, the surfaces of the RHM particles were oxidized to a certain extent so that, when the powder was pressed into various shapes, a high concentration of oxide resulted at the inter-particle or grain boundaries. The inter-granular oxide could be readily attacked by molten aluminum so that the RHM particles or grains could be easily dislodged after molten aluminum attack at the grain boundaries, resulting in the rapid deterioration of the protective RHM cathode surface. During the development work by Ransley, Lewis and others on RHM cathode materials, it was well known that the oxide content of RHM shapes must be kept as low as possible to avoid inter-granular attack by molten aluminum. However, the art of RHM manufacture was not sufficiently advanced at that time to produce high purity RHM products which could withstand attack by molten aluminum for any significant period. Theoretically, RHM with no oxide content at all would be best but such material is impossible to obtain in a commercial process. Lately, several manufacturers have been able to produce relatively large TiB.sub.2 shapes with oxide contents much less than 0.05%, which makes the TiB.sub.2 shapes very resistant to molten aluminum attack even at the grain boundaries where the oxide tends to be concentrated.
Although the RHM products are very strong, they are also quite brittle and subject to thermal shock. As a general rule RHM shapes should not be subjected to a temperature differential greater than 200.degree. to avoid thermal cracking.
A particularly attractive aluminum reduction cell design utilizing RHM cathodic surfaces is shown in U.S. Pat. No. 3,400,061 assigned to the present assignee wherein the RHM cathode surfaces are sloped so that only a thin layer of molten aluminum which wets the RHM surface remains. The molten aluminum electrolytically formed during the operation of the cell drains from the sloped surface into the trough or trench located at the middle of the cell. The molten aluminum in the trough is not a part of the electrolytic circuit and can be removed as desired. Only the thin layer of molten aluminum which wets the RHM cathodic surface is involved in current transfer.
Notwithstanding the suitability of the cell design described in U.S. Pat. No. 3,400,061 a significant problem remained, however, due to the extremely large difference in thermal expansion between RHM shapes and the supporting conductive carbonaceous substrate. The large difference in thermal expansion coefficients (e.g. about 2 .times. 10.sup.-6 v. 8 .times. 10.sup.-6 in/in .degree.F) precluded forming a bond which would be effective both during installation of the RHM shapes at room temperature and the operating temperature of the aluminum reduction cell (e.g., about 975.degree. C). Any bond formed at room temperature when the plate or tiles of RHM were installed would be essentially destroyed by the thermal expansion during heat-up to operating temperature.
The patents and technical literature are replete with references which describe attempts to solve the various problems in the use of TiB.sub.2 and other RHM in the harsh environments of an aluminum reduction cell. Lewis et al. in U.S. Pat. No. 3,400,061 and others, utilized a mixture of TiB.sub.2 and other refractory hard metals with small amounts of carbon to reduce the relatively large thermal expansion of the RHM materials. However, such composites did not have the service life necessary for commercial usage due to their susceptibility of attack by the electrolytic bath. References such as U.S. Pat. Nos. 2,915,442, 3,081,254, 3,151,053, 3,161,579 and 3,257,307, describe RHM cathode bars in various positions. However, the RHM cathode bars usually could not withstand the thermal distortion attendant with such design and they inevitably fractured due to the brittleness of the RHM.
Holliday in U.S. Pat. No. 3,661,736 suggested the use of a composite material comprising fused RHM particles in a binder of carbonaceous material or aluminum carbide as a cathodic surface. It was alleged that a film of aluminum carbide formed on the carbonaceous surface not protected by RHM particles which reduced the attack by the electrolytic bath. However, when aluminum carbide is exposed to molten cryolite, the aluminum carbide is readily dissolved, and in an aluminum reduction cell it is nearly impossible to prevent contact of the cathode surface by cryolite. In the operation of the cell, cryolite would attack the aluminum carbide film as well as the aluminum carbide matrix which holds the particles leading to the early destruction of the cathodic surface.
It is against this background that the present invention was developed.